A raindrop detecting device as shown in FIG. 19 has been proposed as an optical rain sensor for controlling a vehicular wiper. In such a detecting device, the detection area is extremely smaller than the wiping area of a wiper. Therefore, the detection sensitivity should be important performance for the detecting device.
Particularly in the drizzling rain, since the collision probability of raindrops with the detection area is very low, it is difficult for the wiper to be precisely controlled in such a situation.
To solve such a problem, WO91/03393 (National Publication of International Patent Application No. 4-507227) has disclosed a device for controlling a windscreen wiper “by the detection amount of optical energy corresponding to the size of water wall in advance of the windscreen wiper when the windscreen wiper passes through an incident point (detection area)”.
Also, WO91/09756 (National Publication of International Patent Application No. 4-503791) has also disclosed a rainfall response type automatic wiper control system having “means for detecting the presence of waterdrop gathered and carried to a detection region by a wiper”.
Further, the applicant of this invention has also disclosed a similar detecting device in Unexamined Japanese Patent Publication No. 2000-329862.
The techniques disclosed in the above-described three publications are useful as a technique for improving the detection probability of raindrops particularly in case of drizzling rain.
However, the water carried by the wiper is not only raindrops adhering to the wiping region of the wiper. For example, the wiper can carry water collecting in the stop position of the wiper.
If the wiper carries the above-described water after rain has stopped, the following trouble may occur: in such a detecting device, the wiper continues the wiping operation even when the wiping operation is not necessarily needed because rain has stopped.
Also, when a vehicle goes into a tunnel from a rainfall situation, the rainwater adhering to the roof of the vehicle may flow down to the wiping region of a wiper. Further, the rainwater having been wiped away to the outside of the wiping region by the wiper may flow down to the wiping region of the wiper.
In such cases, it is advantageous to wipe the flowing water by at least one wiping operation. In many cases, the visibility is secured by this wiping operation. In the above-described detecting device, however, the wiper is drenched by the wiping operation and carries water, so that there arises a problem in that the wiper continues its wiping operation even after the visibility has been secured.